Programming_Basics
Commands
Prerequisite: Subsystems Exercise
A Command represents an action that the robot can perform.
- Commands are a powerful way to structure robot behaviour.
- Commands can range from simple actions, like moving a motor for a set time, to complex sequences involving multiple subsystems.
- Commands are an implementation of State Machine architecture
Methods of a Command
A command is a special class in WPILib which must contain the following methods:
- initialize: called once when first scheduled, This is where you can set up any necessary parameters or states.
- execute: runs repeatedly (once per cycle), main logic of the command occurs, such as controlling motors or sensors.
- isFinished: checks repeatedly (once per cycle) if exit conditions are met, if ‘true’ command is removed from the scheduler.
- end: called once before the command is removed from schedule (isfinished==true or interrupted), performs transitional actions before entering next state.
Command Scheduler
The Command Scheduler is a core utility in WPILib which orchestrates activity amongst subsystems.
- Manages the execution of the commands, life cycle of the commands(initialize, execute, end).
- Allocates only 1 running command per subsystem. Commands must register subsystems to utilize this functionality.
- Ensures safe execution of the subsytems - by default, commands can’t execute unless robot is active
- Prioritizes Command execution - Commands can be interrupted if a higher priority command is scheduled
Types of Commands
WPILib contains various types of Commands to achieve specific functionality. There are also helpful shortcuts available for implementing common tasks. Heres a shortlist of Command types that are used frequently in Cyber Cat code base:
- Instant Command
- Wait command
- Sequential commands
- Parallel commands
After you complete the following exercise, check out the refernece material at the end of this document to explore these Command types.
Exercise: Create a Command Class to control the position of the Arm
Background: Commands:
- Command requires access to the subsystem which is intended to be controlled, add the subsytem(s)
- Command can access multiple subsystems, but only one command can be active at the given time.
- The addRequirements method plays a crucial role in managing resource usage and ensuring safe access to subsystems.
- addRequirements(), is typically called in the consructor of the command. Bind the commands: Typically in command-based-programming, commands are essentially bound to controller or joystick defining a subsytem -> creating a command -> binding to controller input In this Example, for simulation purposes ‘Default’ command is set on arm subsystem. Default commands: are commands that run continuously for a given subsystem unless interrupted by another command. Default commands allows for automatic and appropriate behavior without explicit scheduling in the robot code. Use cases: Drive train
Start with project from Subsystem Exercise
1) Add a position instance variable to the subsystem(arm).
2) Add a member functions(setter and getter functions) to set and get the position of the arm.
Create a Command class
1) Open project from Simulation section
2) Right-click on subsystems directory and select “Create a new class/commnad”
3) Select Command from the menu and type in a name for the command (e.g. “ArmPrint”) and press enter
4) Create an instance of subsytem(Use the same subsystem(e.g. Arm)ArmSubsystem,
and add it to the command using addRequirements()
5) Set the postion of the arm in the execute() and access the postion variable using getPostion().
Bind the command
1) Add Arm subsysytem instance variable in ‘RobotContainer’.
2) Instantiate the subsystem, and set ‘Default’ command on Arm subsystem in the constructor.
Run Simulation
Simulate the robot code
Choose the ‘Teleoperated’ option in the GUI
Observe the print statements
